Abstract

Older listeners often experience difficulties understanding speech in the presence of background sound. These deficits are thought to reflect neural deficits in the central auditory pathway, as they can occur independent of changes in cochlear hearing thresholds. Here we used a systems-level (scalp recordings) and a microcircuit-level (extracellular recordings) approach in male Fischer-344 rats to investigate how aging affects sensitivity to the temporal envelopes of speech-like sounds in the inferior colliculus. Scalp-recorded potentials suggest an age-related increase in sensitivity to temporal regularity along the ascending auditory pathway. The underlying cellular changes in the midbrain were examined using extracellular recordings from inferior colliculus neurons. We used the local field potential (LFP) as a proxy for a neuron's or neural population's synaptic inputs, and unit activity as a measure of spiking output. We observed an age-related increase in sensitivity to the sound's onset and temporal regularity (i.e., periodicity envelope) in the spiking output of inferior colliculus neurons, relative to their synaptic inputs. This relative enhancement for aged animals was most prominent for multi-unit (in contrast to single-unit) spiking activity. Spontaneous multi-unit, but not single-unit, activity was also enhanced in aged compared to young animals. Our results suggest that aging is associated with altered sensitivity to sound and a sound's temporal regularities, and that these effects may be due to increased gain of neural network activity in the aged auditory midbrain.

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